Marine communications are performed as frequency modulation (FM) communications in the very high frequency (VHF) band. There is currently a demand to miniaturize portable transceivers for marine communications. For such a demand, an internal battery needs to be made small. For example, a single cell battery (3.7 V) is used.
The transceivers for marine communications also require relatively large antenna power (e.g., about 5 W). However, with the above-discussed small power source voltage, a single amplifier circuit using a single power transistor cannot produce the 5 W antenna power. To achieve a high output with low voltage, a structure that connects power amplifier circuits in parallel can be considered, as disclosed by Japanese Laid-Open Patent Application Publication No. H10-322136.
The transceivers for marine communications are required to operate in a high-output mode (or high power mode) with about 5 W but also in a low-output mode (or low power mode) with 1 W or 0.5 W. However, in the structure that connects multiple power amplifier circuits in parallel, as disclosed by JP Laid-Open Publication No. H10-322136, because fluctuations in circuit characteristics overlap for the number of power amplifier circuits, the fluctuations are amplified during the low-output mode, causing unstable operations.
Moreover, recently such transceivers are controlled by an internal central processing unit (CPU). Outputs of the power amplifier circuit are also controlled by approximately 8-bit power control signals output from the CPU. When controlling 0-5 W outputs by 8-bit signals (255 steps), the 1 W output is only ⅕ of the signals (51 steps), and 0.5 W output is only ½ of the ⅕ signals (i.e., about 26 steps). Therefore, during the control of low outputs, resolution degrades due to a reduced number of steps. Thus, the precision control is impossible.
This invention is developed considering such problems and allows to be obtained high power using a low-voltage battery. In addition, this invention provides a high frequency circuit that secures stability and fine control resolution during low-power operation.